1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus or similar electrophotographic image forming apparatus and a process cartridge therefor. More particularly, the present invention relates to an electrophotographic image forming apparatus including an image carrier, latent image forming means for forming a latent image on the image carrier and a developing device for developing the latent image with toner and a process cartridge therefor.
2. Description of the Background Art
It is a common practice with an electrophotographic image forming apparatus to form a latent image on an image carrier, e.g., a photoconductive drum or belt in accordance with image data and develop the latent image with a developing device for thereby producing a corresponding toner image. The image forming apparatus uses either one of a one-ingredient type developer, i.e., toner and a two-ingredient type developer made up of toner and magnetic grains. A developing system of the type using the two-ingredient type developer is desirable in the aspect of image transfer and stability against temperature and humidity. In this type of developing system, the developer rises on a developer carrier in the form of a magnet brush in a developing zone where the developer carrier faces the image carrier, so that the toner is fed from the magnet brush to the latent image.
However, a conventional developing system using the tow two-ingredient type developer has a problem to be described later in detail.
Japanese Patent Laid-Open Publication No. 9-152778, for example, discloses a developing device in which an auxiliary magnet faces a magnetic pole configured to retain a developer. The auxiliary magnet, which is of the same polarity as the magnetic pole, is positioned outside of a casing or storing means for storing a developer. In this configuration, the frictional resistance of the bottom of the casing acting on the developer is reduced to insure the fluidity of the developer collected after development, thereby preventing the developer from accumulating on the bottom of the casing. This kind of scheme, however, is not directed toward the collection of a developer falling off the developer carrier. Further, the magnetic force of the auxiliary magnet facing the developer retaining pole is likely to decrease.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication No. 8-30098 and Japanese Patent Publication Nos. 6-64396 and 8-33692.
It is an object of the present invention to provide an image forming apparatus capable of preventing a developer falling off the developer carrier from escaping the developing device and a process cartridge therefor.
An image forming apparatus of the present invention includes an image carrier, a latent image forming device for forming a latent image on the image carrier, and a developing device for developing the latent image to thereby produce a corresponding toner image. The developing device includes a casing for storing a developer consisting of toner and magnetic grains and is formed with an opening facing the image carrier. A rotatable developer carrier is supported in the casing and formed of a nonmagnetic material. A magnetic field generating device causes the developer to rise in the form of a magnet brush on the developer carrier in a developing zone that faces the image carrier. The magnetic field generating device is implemented by a main magnetic pole positioned inward of the developer carrier with respect to the developing zone and an auxiliary magnetic pole positioned downstream of the main magnetic pole in the direction of rotation of the developer carrier and opposite in polarity to the main magnetic pole. A magnetic field forming member is disposed in the casing for forming a magnetic field that prevents the developer falling off the developer carrier from escaping the developing device via the opening. The magnet brush on the developer carrier is moved in the same direction as the surface of the image carrier, as viewed in the developing zone, but at a higher speed than the surface of the image carrier in contact with the surface of the image carrier, thereby developing the latent image.